Interferometric electrooptic modulators fabricated in the substrate material lithium niobate (LN) are widely used in digital communication systems operating at 2.5 Gb/s and 10 Gb/s and in analog systems for cable television. Not only are modulator rise and fall times &lt;10 ps achieved with this technology, but interferometric designs provide the chirp free performance needed for long-distance transmission. These devices utilize a traveling wave (TW) configuration in which the modulating microwave signal propagates in a strip line or coplanar waveguide on the surface of the insulating substrate in the same direction as the modulated light wave, as described by G. K. Gopalakrishnan et al. in Journal of Lightwave Technology, vol 12, pp. 1807-1818, 1994. Best performance for high speed or high bandwidth modulation is achieved if the velocity of the modulating radio frequency wave closely matches that of the modulated optical wave. Present practice for the highest bandwidths (&gt;&gt;1 GHz) is to use very thick (.apprxeq.15-30 .mu.m) electrodes to achieve velocity matching by increasing the microwave propagation speed to match that of the optical carrier.
In spite of recent commercial success, the present TW modulator technology still has some shortcomings. Electrical power required to drive the modulators at microwave frequencies is high (typically several hundred mW for a pi-radian phase retardation). This means that a medium power microwave amplification circuit is needed in each transmitter. In the case of analog transmission, the relatively low sensitivity of modulated power to applied voltage and the inherent nonlinearity in dependence of modulated power on applied voltage can adversely affect link dynamic range. Further, the requirement for very thick electrodes on the LN substrate substantially increases the fabrication cost of the modulator chip.
One approach to overcoming these shortcomings is to use a material which supports a stronger electrical/optical interaction. Ferroelectric materials such as strontium barium niobate (SBN) with much higher electrooptic coefficients than LN have been known for decades, and low-loss waveguides and GHz-bandwidth modulation have recently been demonstrated in such materials. However, it is well known that materials with such high electrooptic coefficients also have very large dielectric constants. This means that microwave propagation is very slow, so that prohibitively thick electrodes are needed for velocity matching by the conventional method.